1. Field of the invention
The present invention relates to an optical transmitter for use in an optical transmission system, and more specifically to an optical transmitter provided with a packaging structure by which excellent optical transmitting waveform at high speed transmission can be obtained.
2. Prior Art
Recently, an optical transmission technique has become to be used commonly in data communicating system, therefore, an optical transceiver which is a key device of an optical transmission has become to be used in large quantities.
On the other hand, since transmitting speed required to an optical transceiver has become to be high more and more, in future, it seems that an optical transceiver having transmitting speed 10 Gbit/s or more than 10 Gbit/s will become to be used commonly in large quantities. Therefore, techniques to realize high speed transmitting with low cost are required strongly more than former.
To obtain a high speed optical transmitter, speed up of an optical element itself such as laser diode (LD) or outside modulator is important, but packaging method of an optical element is one of key points, too. Namely, it is necessary to devise a packaging structure which can obtain excellent optical transmitting waveform at high speed transmission, and to make it a realizable structure with low cost.
In FIG. 1(a), a front view of a conventional optical transmitter is shown, and in FIG. 1 (b), a plane view of it is shown.
AD shown in FIG. 1(a) and in FIG. 1(b), a conventional optical transmitter comprising mainly, the LD chip 30 which outputs light: the semicylindrical column 11 provided on its side with an overhang potion having supporting surface of the LD chip 30, the disk stem 10 supporting the LD chip 30 vertically to coincide the optical axis of the LD chip 30 with the axis of the optical fiber to which output light it input, and three terminals 12, 13 and 14 provided through the stem 10. Further, either electrode of the LD chip 30 is connected with the drive terminal 14 by a bonding wire 15, and another electrode of the Lo chip 30 is connected electrically with the common terminal 13 through the conductive stem 10 and column 11.
Further more, the optical transmitter is provided with the monitoring PD chip 31 which monitors emitting power of output light from the LD chip 30, on the stem 10 locating under the overhang potion, either electrode of the monitoring PD chip 31 is connected with the PD terminal 12, and another electrode of the monitoring PD chip 31 is connected with the common terminal 13 by a bonding wire 16.
These articles provided on the stem 10 are covered by the cap 20 equipped upper of it with the lens 21 which passes output light from the LD chip 30.
The above explained packaging structure is, at present, the most generally used package of light emitting element, and is called TO can package.
The stem 10 and the cap 20 are formed by metal, and the LD chip 30 is shielded hermetically, therefore, the package is very reliable.
Further, since the package has already been used enormously, automatic assembling is progressing and exceeding in cost.
Further more, since this package is coaxial in shape and emits light to axial direction, it can be applied easily to a receptacle type optical transmitter to which an optical connector is inserted directly.
However, in the conventional technique, because the connection between the LD chip 30 and the drive terminal 14 is made by wire bonding 15, the bonding wire 15 causes parasitic inductance which lowers transmitting speed. Although length of the bonding wire 15 is designed to be as short as possible, but limitation to shorten the length is existing, problem has been arisen that it is difficult to catch up with demand for high speed transmission more and more recently.
Further, this parasitic inductance causes disturbance of frequency characteristic, it makes speedup difficult more and more to provide resistance outside of the package so as to dump the disturbance.
Further more, since parasitic inductance of the bonding wire is varies according to a degree of strain or so on, as shown in FIG. 2, impedance between the LD chip 30 and the drive terminal 14 varies with every product, there is disadvantage that deteriorates reproducibility and causes to lower yield rate.
Except TO can package, various devises has been made to improve high frequency characteristic, there is a great problem that cost is very expensive.
Recently, receptacle type optical transmitter with easy handling is required strongly for high speed optical transmitter, there has not been realized TO can package applicable to such optical transmitter that operates at high speed.
As described above, conventional packaging method of light emitting element is required to improve stability of wideband characteristic and frequency characteristic.
An object of the present invention is to provide an optical transmitter provided with reduced parasitic inductance by shortening bonding wire, and high speed transmission and stable frequency characteristic with low cost.
In accordance with this invention, there is provided an optical transmitter, comprising, a stem formed almost coaxial shape, a column fixed on said stem, a light emitting element chip provided on said column, terminals insulated with said stem and provided through said stem, and a connecting article connecting between said terminal and said light emitting element chip, wherein, said connecting article is formed by a line which is provided on a insulating substrate and is controlled to have predetermined characteristic impedance, and a resistive element for impedance matching connected to said line serially.
In accordance with this invention, there is provided an optical transmitter, wherein, said resistive element for impedance matching is provided with matching resistor formed directly on said insulating substrate or chip resistor.
In accordance with this invention, there is provided an optical transmitter, wherein, a branched wiring is provided from middle of said matching resistor or chip resistor and said light emitting element chip, and said branched wiring is connected with a terminal which is connected electrically with outside.
In accordance with this invention, there is provided an optical transmitter, wherein, said branched wiring is provided with at least resistor and inductor so as to attenuate high frequency signal.
In accordance with this invention, there is provided an optical transmitter, wherein, diameter of said terminal, diameter of a hole formed through said stem to pass said terminal, and dielectric constant of insulator which is embedded to fix said terminal within said hole are controlled to make almost equalize characteristic impedance of said line with impedance of said terminal.
In accordance with this invention, there is provided an optical transmitter, wherein, sum of impedance of said light emitting element chip and impedance of said matching resistor or said chip resistor equals characteristic impedance of said line.
In accordance with this invention, there is provided an optical transmitter, comprising, a stem formed almost coaxial shape, a column fixed on said stem, an outside modulating element provided on said column, terminals insulated with said stem and provided through said stem, and a connecting article connecting between said terminal and said outside modulating element, wherein, said connecting article is formed by a line which is provided on a insulating substrate and is controlled to have predetermined characteristic impedance, and a resistive element for impedance matching connected to said line serially.
In accordance with this invention, there is provided an optical transmitter, wherein, said resistive element for impedance matching is provided with matching resistor formed directly on said insulating substrate or chip resistor.
In accordance with this invention, there is provided an optical transmitter, wherein, a branched wiring is provided from middle of said matching resistor or chip resistor and said light emitting element chip, and said branched wiring is connected with a terminal which is connected electrically with outside.
In accordance with this invention, there is provided an optical transmitter, wherein, said branched wiring is provided with at least resistor and inductor so as to attenuate high frequency signal.
In accordance with this invention, there is provided an optical transmitter, wherein, diameter of said terminal, diameter of a hole formed through said stem to pass said terminal, and dielectric constant of insulator which is embedded to fix said terminal within said hole are controlled to make almost equalize characteristic impedance of said line with impedance of said terminal.
In accordance with this invention, there is provided an optical transmitter, wherein, sum of impedance of said outside modulating element and impedance of said matching resistor or said chip resistor equals characteristic impedance of said line.